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| United States Patent |
5,318,033
|
|
Savord
|
June 7, 1994
|
Method and apparatus for increasing the frame rate and resolution of a
phased array imaging system
Abstract
Scan conversion or data interpolation is performed on the signal generated
by acoustic transducers in an acoustic imaging system before the signal is
processed by detecting and limiting it. This processing uses signal phase
information, which is normally lost during the image reconstruction
process, to increase image resolution. A nonlinear interpolation scheme is
used during the scan conversion process used to convert the data generated
by acoustic transducers into data suitable for visual display in order to
more accurately generate interpolated data points. A nonlinear angular
spacing between acoustic lines is used to increase the image frame rate
can be increased without decreasing image resolution. The frame rate of
the acoustic imaging system can also be increased by interpolating the
signals generated by the transducers before they are provided to
beamforming circuits.
| Inventors:
|
Savord; Bernard J. (Andover, MA)
|
| Assignee:
|
Hewlett-Packard Company (Palo Alto, CA)
|
| Appl. No.:
|
870388 |
| Filed:
|
April 17, 1992 |
| Current U.S. Class: |
600/447; 73/625 |
| Intern'l Class: |
A61B 008/00 |
| Field of Search: |
128/660.07,661.01,661.09
73/625
364/413.25,514,577
|
References Cited
U.S. Patent Documents
| 4817617 | Apr., 1989 | Takeuchi et al. | 128/661.
|
| 5027821 | Jul., 1991 | Hirama et al. | 128/661.
|
| 5127409 | Jul., 1992 | Daigle | 128/660.
|
Primary Examiner: Jaworski; Francis
Claims
What is claimed is:
1. Apparatus for reducing image artifacts in a phased array acoustic
imaging system having a plurality of acoustic transducer elements, means
connected to said plurality of transducer elements for generating a
transmit acoustic beam for interrogating an object, means connected to
said plurality of transducer elements for receiving signals from a receive
acoustic beam and means responsive to received acoustic signals for
generating image signals in a first format, said apparatus comprising:
means for generating a plurality of image display signals by selectively
delaying said image signals by predetermined time intervals which time
intervals are a multiple of the time interval necessary to generate a
transmit acoustic beam and receive signals reflected from said object; and
means for multiplying each of said plurality of image display signals by a
predetermined constant value to generate a plurality of sealed image
display signals wherein said predetermined constant values add to a total
greater than one; and
means for adding together each of said plurality of scaled image display
signals to generate image display signals in a second format.
2. Apparatus for reducing image artifacts according to claim 1 wherein said
predetermined constant values are set to the normalized values of a sin
x/x function between the values x=0 and x=.pi..
3. Apparatus for reducing image artifacts according to claim 1 wherein
there are two image display signals and said predetermined constant values
are each 0.58.
4. A method for reducing image artifacts in a phased array acoustic imaging
system having a plurality of acoustic transducer elements, means connected
to said plurality of transducer elements for generating a transmit
acoustic beam for interrogating an object, means connected to said
plurality of transducer elements for receiving signals from a receive
acoustic beam and means responsive to received acoustic signals for
generating image signals in a first format, said method comprising the
steps of:
A. generating a plurality of image display signals by selectively delaying
said image signals by predetermined time intervals which time intervals
are a multiple of the time interval necessary to generate a transmit
acoustic beam and receive signals reflected from said object;
B. multiplying each of said plurality of image display signals by a
predetermined constant value to generate a plurality of scaled image
display signals wherein said predetermined constant values add to a total
greater than one; and
C. adding together each of said plurality of scaled image display signals
to generate image display signals in a second format.
5. A method for reducing image artifacts according to claim 4 wherein step
B comprises the steps of:
B1. adjusting said predetermined constant values to the normalized values
of a sin x/x function between the values x=0 and x=.pi..
6. Apparatus for increasing the frame rate of a phased array acoustic
imaging system having a plurality of acoustic transducer elements, means
connected to said plurality of transducer elements for sequentially
generating transmit acoustic beams for interrogating an object, said
apparatus comprising:
interpolator means connected to each of said plurality of transducer
elements, said interpolator means being responsive to receive signals
generated by said each transducer element from acoustic energy from at
least two sequential transmit beams for generating at least two
interpolated outputs;
a plurality of beamformer means responsive to interpolated outputs
generated by said interpolator means for generating image data
corresponding to at least two receive beams; and
means responsive to said image data for generating a visual display of said
object.
7. Apparatus for increasing the frame rate of a phased array acoustic
imaging system according to claim 6 wherein said interpolator means
comprises:
means responsive to receive signals generated by said each transducer
element for temporarily storing said receive signals for at least two
different time periods;
means responsive to stored receive signals for scaling each of said stored
receive signals by a predetermined constant; and
means for summing the scaled receive signals to generate an interpolated
output.
8. Apparatus for increasing the frame rate of a phased array acoustic
imaging system according to claim 7 wherein said interpolator means
further comprises:
means responsive to stored receive signals for scaling each of said stored
receive signals by at least two predetermined constants to generate at
least two intermediate outputs; and
means for summing said intermediate outputs to generate at least two
interpolated outputs.
9. Apparatus for increasing the frame rate of a phased array acoustic
imaging system according to claim 6 wherein each beamformer means has a
plurality of inputs, each of said plurality of inputs corresponding to one
of said plurality of transducer elements.
10. Apparatus for increasing the frame rate of a phased array acoustic
imaging system having a plurality of acoustic transducer elements, means
connected to said plurality of transducer elements for sequentially
generating transmit acoustic beams for interrogating an object, said
apparatus comprising:
storage means connected to each of said plurality of transducer elements,
said storage means being responsive to receive signals from said
transducer elements for generating at least two stored outputs
representing receive signals generated by said each transducer element
from acoustic energy from at least two sequential transmit beams;
first means for scaling said stored outputs by a first set of predetermined
constants to generate a first set of intermediate outputs;
second means for scaling said stored outputs by a second set of
predetermined constants to generate a second set of intermediate outputs;
first summing means for summing said first set of intermediate outputs to
generate a first interpolated output;
second summing means for summing said second set of intermediate outputs to
generate a second interpolated output;
a plurality of beamformer means responsive to said first set of
interpolated outputs and said second set of interpolated outputs for
generating image data corresponding to at least two receive beams;
means responsive to said image data for generating a visual display of said
object.
11. Apparatus for increasing the frame rate of a phased array acoustic
imaging system according to claim 10 wherein said storage means comprise a
plurality of delay means.
12. Apparatus for increasing the frame rate of a phased array acoustic
imaging system according to claim 10 wherein said first scaling means
comprises a first plurality of multipliers, one of said first plurality of
multipliers multiplying one of stored outputs by a first predetermined
constant.
13. Apparatus for increasing the frame rate of a phased array acoustic
imaging system according to claim 10 wherein said second scaling means
comprises a second plurality of multipliers, one of said second plurality
of multipliers multiplying one of stored outputs by a second predetermined
constant which differs from said first predetermined constant.
14. A method for increasing the frame rate of a phased array acoustic
imaging system having a plurality of acoustic transducer elements, means
connected to said plurality of transducer elements for sequentially
generating transmit acoustic beams for interrogating an object, said
method comprising the steps of:
A. generating at least two interpolated outputs from receive signals
generated by said each transducer element from acoustic energy from at
least two sequential transmit beams;
B. generating image data corresponding to at least two receive beams from
said interpolated outputs generated in step A; and
C. generating a visual display of said object from said image data
generated in step B.
15. A method for increasing the frame rate of a phased array acoustic
imaging system according to claim 14 wherein step A comprises the steps
of;
A1. temporarily storing said receive signals for at least two different
time periods to generate stored receive signals;
A2. scaling each of said stored receive signals by a predetermined
constant; and
A3. summing the scaled receive signals to generate an interpolated output.
16. A method for increasing the frame rate of a phased array acoustic
imaging system according to claim 15 wherein step A2 comprises the steps
of:
A2A. scaling each of the stored receive signals with at least two
predetermined constants to generate at least two intermediate outputs; and
A2B. summing said intermediate outputs to generate at least two
interpolated outputs.
17. A method for increasing the frame rate of a phased array acoustic
imaging system according to claim 14 wherein step B comprises the steps
of:
B1. selectively delaying said interpolated outputs; and
B2. summing said selectively displayed interpolated outputs to form said
image data.
18. A method for increasing the frame rate of a phased array acoustic
imaging system according to claim 14 wherein step C comprises the steps
of:
C1. converting said image data generated in step B into a data format
suitable for visual display.
19. A method for increasing the frame rate of a phased array acoustic
imaging system having a plurality of acoustic transducer elements, means
connected to said plurality of transducer elements for sequentially
generating transmit acoustic beams for interrogating an object, said
method comprising the steps of:
A. generating receive beam data for at least two received beams from
receive signals generated by said each transducer element from acoustic
energy from each transmit beam;
B. generating image data from said at least two receive beams generated in
step A; and
C. generating a visual display of said object from said image data
generated in step B;
wherein step B comprises the steps of:
B1. temporarily storing said receive beam signals for at least two
different time periods to generate stored receive beams signals;
B2. scaling each of said stored receive beam signals by a predetermined
constant; and
B3. summing the scaled receive beam signals to generate an interpolated
output;
wherein step B2 comprises the steps of:
B2A. scaling each of the stored receive beam signals with at least two
predetermined constants to generate at least two intermediate outputs; and
B2B. summing said intermediate outputs to generate at least two
interpolated outputs.
20. A method for increasing the frame rate of a phased array acoustic
imaging system according to claim 19 wherein step C comprises the steps
of:
C1. selectively delaying said interpolated outputs; and
C2. summing said selectively displayed interpolated outputs to form visual
image data.
21. A method for increasing the frame rate of a phased array acoustic
imaging system according to claim 20 wherein step C further comprises the
steps of:
C3. converting said visual image data generated in step C2 into a data
format suitable for visual display.
Description
FIELD OF THE INVENTION
This invention relates to phased-array acoustic systems and, in particular,
to ultrasonic phased-array imaging systems.
BACKGROUND OF THE INVENTION
Ultrasonic imaging systems for producing real-time images of internal
portions of the human body are well-known. In one such system, an array of
ultrasonic transducers placed in contact with the body converts short
electrical pulses into corresponding pressure waves. The electrical pulses
can be applied to each individual transducer in the array and by choosing
the application time of the pulses to each transducer relative to the
other transducers in the array, the pressure waves generated by each
transducer can be formed into a "transmit beam" which propagates in a
predetermined direction from the array.
As the pressure waves in the transmit beam pass through the body, a portion
of the acoustic energy is reflected back towards the transducer array
whenever the waves encounter tissues having different acoustic
characteristics. An array of receiving transducers (which may be the same
as the transmitting array) is provided for converting the reflected
pressure pulses into corresponding electrical pulses. The reflected
pressure pulses are received by each transducer in the receiving array and
by suitably choosing relative delays between the signals generated by each
transducer and combining the signals, the received pressure waves located
in a "receiving beam" can be emphasized preferentially to other pressure
pulses. As with the transmit beam, the relative transducer delays can be
adjusted so that the receiving beam extends in any desired direction from
the transducer array.
It is also possible to "focus" the received acoustic signals at a point
along the receiving beam. This is done by selectively adjusting relative
signal delays between the transducers so that the electrical signals
generated by the receiving transducers are superimposed in time for
signals received from a point along the receiving beam at a predetermined
distance from the transducer array, but are not superimposed for other
signals. Consequently, when the signals are combined, a strong signal is
produced from signals corresponding to this point whereas signals arriving
from other points at different times have random phase relationships and
therefore destructively interfere.
A two-dimensional image plot or sector image can be generated with this
system by adjusting the acoustic transducers to generate or "shoot" a
transmit beam at a selected angular direction from the transducer array.
The receiving transducers are then adjusted to generate a receiving beam
at the same angle as the transmitting beam. The receiving transducers are
adjusted to focus the receiving beam at sequentially increasing distances
from the transducer array along the predetermined transmit beam angle. The
received signals for each sequential focal point are stored. The transmit
and receive beams are then moved by a predetermined angular amount and the
process of acquiring signals is repeated. The started signals are then
processed to generate a wedge-shaped acoustic image called a sector.
Since the distances between any desired focal point along the receiving
beam and the various receiving transducers are different, the reflected
pressure pulses arrive at the transducers at different times, thereby
generating electrical signals at different times. It is therefore
necessary to introduce compensating electrical delays between each
transducer and the signal summing point so that the time of arrival of all
of the electrical signals at the summing point is the same regardless of
which transducer is involved. The collection of transducer compensating
delays and the signal summing circuitry is normally referred to as a
"beamformer" and is described, for example, in U.S. Pat. No. 4,140,022
issued to the assignee of the present invention. The description of the
beamformer apparatus described therein is hereby incorporated by
reference.
The output of the beamformer is generally a radio-frequency signal
representing the amplitude of the received pressure pulses. The signals
are often a function of the angle (.theta.) of the receive beam and the
radial distance (R) along the receive beam at which the focal point
occurs. Consequently, the signals are said to be in R-.theta. coordinates.
It is also possible, using conventional construction methods, to construct
a beamformer which generates scanning information in other coordinate
systems, such as a linear scan. However, by considering small, localized
areas, signals expressed in these other coordinate systems can be
converted to R-.theta. coordinates. Therefore, the following discussion
will assume R-.theta. coordinates without loss of generality.
Generally, the signals are displayed on a display monitor such as a
television or raster-scan monitor and, thus, the format of the signals
must be converted from R-.theta. coordinates to the X-Y coordinates used
in the television display. This conversion is performed by a device called
an X-Y scan converter. Since actual data is available in R-.theta.
coordinates at discrete angular positions, the scan converter must
generate the required X-Y values by interpolating between the R-.theta.
coordinate values. The construction and operation of such scan converters
is well-known. For example, scan converters are discussed in detail in
U.S. Pat. Nos. 4,468,747 and 4,471,449, both assigned to the assignee of
the present invention. The description of these patents is hereby
incorporated by reference and, accordingly, the detailed construction of
scan converters will not be discussed further herein.
It has been found that with some conventional scan converter systems
certain problems occur. One such problem is that the images produced by
the system often have "artifacts" in the reconstructed image. Artifacts
are visual anomalies that appear in the displayed image but are not
present on the actual object. Such anomalies may consist of radiating
lines, checkerboard patterns or speckles and are generally related to
imperfect reconstruction of the image.
Another problem with prior art systems is that they often have limited
resolution. One known method of increasing image resolution is to increase
the number of acoustic lines which are shot by reducing the angular
increment between lines. Obviously, such an approach increases the overall
time necessary to obtain the acoustic data and reconstruct the image.
Since many ultrasonic imaging systems are used for imaging moving objects
such as heart valves, it is of prime importance to generate an image as
fast as possible (by increasing the "frame rate" or the number of images
generated per unit time) so that the object motion can be depicted as
accurately as possible. The frame rate can be increased by decreasing the
number of lines which are shot to produce each image. However, as
previously discussed, this also reduces the overall resolution of the
image. Consequently, in prior art systems there has been a trade-off
between resolution and frame rate.
Accordingly, it is an object of the present invention to provide a method
and apparatus for increasing resolution without correspondingly reducing
the frame rate of the system.
It is a further object of the present invention to increase the
signal-to-noise ratio of the system or increase the frame rate without
correspondingly increasing the amount of circuitry or time involved
generating an acoustic image.
It is a further object of the present invention to utilize additional
information normally discarded during the prior art reconstruction process
to provide better resolution upon reconstruction.
It is a further object of the present invention to reduce artifacts in the
acoustic image produced by imperfect prior art reconstruction processes.
It is still a further object of the present invention to increase the
resolution without increasing the acoustic line density.
SUMMARY OF THE INVENTION
The foregoing objects are achieved and the foregoing problems are solved in
one illustrative embodiment of the invention in which an acoustic imaging
system is treated as a Nyquist sampled data system. In accordance with one
aspect of the invention, the signal processing order is changed in order
to use signal phase information, which is normally lost during the
reconstruction process, to increase image resolution. In particular, in
the inventive system and method, scan conversion or data interpolation is
performed on the signal generated by the transducers before the signal is
processed by detecting and limiting it.
In accordance with another aspect of the invention, a nonlinear
interpolation scheme is used during the scan conversion process to convert
the R-.theta. data into X-Y data. It has been found that prior art linear
interpolation underestimates the image intensity between data and that a
nonlinear interpolation reduces the underestimation. More particularly, in
accordance with the invention, the image data value between two lines is
estimated by using an interpolator which estimates the data in accordance
with numerical values which describe the main lobe of a sinc (sin x/x)
function.
In an additional embodiment of the invention, it has been found that, for
imaging systems which use linear transducer arrays, when the transmit and
receive beams are directed to a position which forms a large angle
relative to a line perpendicular to the array, the effective phased array
aperture is reduced by the cosine of the beam or steering angle. This
reduction results in a wider beam width. Accordingly, a larger angular
spacing between acoustic lines can be used to obtain the same image
resolution as an image obtained with equal line spacing between acoustic
lines. In turn, this wider spacing reduces the number of lines which are
required to be used at large angles in order to obtain a predetermined
image resolution. Therefore, the image frame rate can be increased without
decreasing image resolution. In particular, it has been found that
acoustic lines spaced on a grid uniform in the reciprocal of the cosine of
the steering angle produce satisfactory results.
In accordance with still another aspect of the present invention, the frame
rate of an acoustic imaging system is increased by interpolating the
signals generated by the transducers before they are provided to the
beamforming circuit. More particularly, the angular separation between
acoustic lines is increased to reduce the number of lines shot, thereby
increasing the frame rate. The corresponding loss of resolution which
would then normally occur is prevented by synthesizing the image
information which would normally be contained in the missing acoustic
lines by interpolating the existing data for angular positions between
existing lines.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified block electrical schematic diagram of a prior art
acoustic imaging system;
FIG. 2 is a more detailed block electrical schematic diagram of the prior
art scan converter circuit illustrated in FIG. 1;
FIG. 3 is a simplified block electrical schematic diagram of an acoustic
imaging system in which the apparatus has been reorganized in accordance
with one aspect of the invention so that scan conversion is performed
prior to signal detection and logging in order to increase the image
resolution;
FIG. 4A is a cross-sectional view of an acoustic imaging test apparatus in
which two "positive" target wires are embedded in gelatin material and
used to illustrate the image improvement obtained with the apparatus of
FIG. 3.
FIG. 4B is a cross-sectional view of an acoustic imaging test apparatus in
which one "positive" target wire and one "negative" target wire are
embedded in gelatin material and used together with FIG. 4A to illustrate
the image improvement obtained with the apparatus of FIG. 3.
FIG. 5A illustrates the signal amplitude generated by the acoustic
transducers versus receive beam angle for the test setup illustrated in
FIG. 4A using the prior art imaging apparatus shown in FIG. 1.
FIG. 5B illustrates the signal amplitude generated by the acoustic
transducers versus receive beam angle for the test setup illustrated in
FIG. 4B using the prior art imaging apparatus shown in FIG. 1.
FIG. 6A illustrates a signal which results from the detection of the signal
in FIG. 5A.
FIG. 6B illustrates a signal which results from the detection of the signal
in FIG. 5B.
FIG. 7A illustrates a signal which results from sampling the signal in FIG.
6A.
FIG. 7B illustrates a signal which results from sampling the signal in FIG.
6B.
FIG. 8A illustrates a signal which results from scan conversion or linear
interpolation of the signal in FIG. 7A.
FIG. 8B illustrates a signal which results from scan conversion or linear
interpolation of the signal in FIG. 7B.
FIG. 9A illustrates a signal which results from sampling the signal in FIG.
5A in accordance with the apparatus shown in FIG. 3.
FIG. 9B illustrates a signal which results from sampling the signal in FIG.
5B in accordance with the apparatus shown in FIG. 3.
FIG. 10A illustrates a signal which results from scan conversion or linear
interpolation of the signal in FIG. 9A.
FIG. 10B illustrates a signal which results from scan conversion or linear
interpolation of the signal in FIG. 9B.
FIG. 11A illustrates a signal which results from the detection of the
signal in FIG. 9A.
FIG. 11B illustrates a signal which results from the detection of the
signal in FIG. 9B.
FIG. 12 shows a magnified image of a section of heart muscle generated
using a conventional ultrasonic imaging system.
FIG. 13 shows a magnified image of the section of heart muscle shown in
FIG. 11 generated using a the inventive ultrasonic imaging system.
FIG. 14 is a block schematic diagram of a preferred circuit structure for
performing scan conversion interpolation.
FIG. 15 is a graphical diagram illustrating an interpolation function in
accordance with one aspect of the invention.
FIG. 16 is a schematic illustration indicating a prior art method of
shooting acoustic lines at equal angular increments;
FIG. 17 is a schematic illustration of a scan sequence in accordance with
the present invention in which the scan lines are shot at unequal
intervals;
FIG. 18 illustrates a method for increasing system frame rate by
synthesizing receive information prior to beamforming.
FIG. 19 is a schematic diagram indicating a conventional connection of
transducer elements to the beamformer;
FIG. 20 is a schematic diagram in accordance with another aspect of the
present invention in which the acoustic elements are connected to a
plurality of beamformers by means of interpolation circuits;
FIG. 21 is a modification of the circuitry shown in FIG. 20 in which a
plurality of interpolation circuits is connected to the output of a
plurality of beamformers in order to reduce the number of acoustic scan
lines necessary to reconstruct the image.
FIG. 22 is a schematic illustration of synthesized receive beam information
in relation to transmit beam information using circuitry such as that
shown in FIGS. 20 and 21 and received information from three parallel
beams.
FIG. 23 is a schematic illustration of synthesized receive beam information
in relation to transmit beam information using circuitry such as that
shown in FIGS. 20 and 21 and received information from four parallel beams
.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
FIG. 1 is a simplified block schematic diagram of a conventional
phased-array acoustic imaging system. In particular, at the left side of
the figure, an array of transducers 100 is connected to the input of a
beamformer circuit 102 (only a single transducer is depicted for clarity).
In general, the same array of transducers is used to both generate the
transmit beam as well as receive the reflected pressure pulses. Although
transducer 100 is schematically shown connected directly to beamformer
102, in actuality, transmit drivers and receive amplifiers would be
connected between the transducers and the beamformer. The construction and
connection of these latter circuits is well-known and, consequently, they
are omitted from FIG. 1 for clarity.
The construction and operation of a beamformer circuit is also well-known
to those skilled in the art and is discussed in more detail in the
afore-mentioned U.S. Pat. No. 4,140,022. Briefly, the circuit contains a
plurality of delay lines for selectively delaying transducer signals and a
summing network to combine the delayed signals to produce an output
electrical signal on line 104.
The beamformer output on lead 104 (as previously mentioned this output is
in R-.theta. coordinates) is then processed to generate the final X-Y
signals which can be displayed on T.V. display 112. In particular, the
output on lead 104 is detected and compressed prior to providing it to a
scan converter which converts the R-.theta. coordinates to X-Y
coordinates. This additional processing is generally necessary because the
beamformer output signals have a large dynamic range whereas a typical TV
monitor can only display signals of a very limited dynamic range.
Accordingly, the beamformer output signal on lead 104 is applied to a
detector circuit 106.
Detector circuit 106 is typically an "absolute value" or "square-law" type
detector which has been schematically illustrated in FIG. 1 as a diode. As
the construction and operation of such detectors is well-known, detector
106 will not be discussed further herein, but the detector will be assumed
to be an absolute value detector. The output of detector 106 is a signal
which contains a DC level related to the magnitude of the input signal.
This latter signal is provided to amplifier 108.
Amplifier 108 is used to reduce the dynamic range of the signal generated
by detector 104 to the signal range that can be handled by the TV monitor
112. A typical device is a logarithmic amplifier called a "logger" which
generates the output log(x) in response to an input signal x. However,
other data compression devices are known and could be substituted for the
logarithmic amplifier. Such devices might include any type of amplifier
with a nonlinear transfer characteristic. The construction and operation
of such data compression devices are conventional and will not be
discussed further herein.
The output of amplifier 108 is provided to scan converter 110 which
converts the scan data in R-.theta. coordinates to the X-Y coordinates
needed for display. In general, the construction and operation of a scan
converter schematically illustrated as box 110 is well-known. A more
detailed block diagram is also shown in FIG. 2 which depicts the
construction of the circuitry that performs the interpolation necessary to
convert the R-.theta. signals to X-Y signals.
R-.theta. data on line 200 from data compression device 108 is provided
directly to a scaling circuit 202 which multiplies the data by a
preselected constant (A). Incoming data on input line 200 is also provided
to a "one-line" buffer 204. In the case of analog data, buffer 204 may be
a simple delay line which delays the analog information from line 200 for
a time interval equal to the time delay between acoustic lines generated
by the transducer array. Alternatively, if the incoming signals have been
digitized, buffer 204 may be a temporary memory. In any case, the output
of buffer 204 is provided to a second scaling device, 206, which scales
the information by a second predetermined constant. Buffer 208 of scaling
device 202 and output 210 of scaling device 206 are provided to a summing
network 212 which produces the output 214. The buffer 204 allows the
circuit to generate an interpolated value of the data for points occurring
between scan lines. The output of scan converter 110 is provided to a TV
monitor 112 for display.
In accordance with the invention, the resolution of an acoustic image
generated by an imaging system such as shown in FIG. 1 can be
significantly increased by changing the signal processing order. In
particular, as shown in FIG. 3, if scan conversion is performed on the
data before detection and compression, the resolution of the image can be
enhanced without increasing the number of scan lines. In particular, in
FIG. 3, transducer 300 and beamformer 302 correspond to elements 100 and
102, respectively, shown in FIG. 1. The data signal generated by
beamformer 302 on lead 304 is provided directly to scan converter 312
instead of detector 306 as in the prior art structure. The output of scan
converter 310 is, in turn, provided to detector 306 and data compression
device 308 and the output of amplifier 308 is provided to TV monitor 312
for display.
The effect of the inventive change in processing order can be explained by
referring to FIGS. 4-11. FIGS. 4A and 4B illustrate a conventional manner
of testing an acoustic imaging apparatus using a test "phantom" device in
which "targets" formed by metal wires are embedded in a gelatin material
that has an acoustic impedance approximately equal to that of water. FIGS.
4A and 4B illustrate two separate cross-sectional diagrams through two
phantoms in a direction perpendicular to the wire axis. A wire can be
either "positive" (depicted as a "+" sign) indicating that it has an
acoustic impedance greater than water or the wire can be "negative"
(depicted as a "-" sign) indicating that it has an acoustic impedance less
than water. These wires are then imaged by generating acoustic beams and
sweeping them across the wires.
In FIG. 4A, two acoustic lines are shown interrogating, or locating the
position of, two positive wires. Dotted line 400 illustrates an acoustic
transmit beam shot to interrogate wire 402 and dotted line 404 represents
a transmit beam used to interrogate wire 406. FIG. 4B illustrates a second
phantom in which acoustic lines are shot to interrogate one positive and
one negative wire. Line 408 represents a transmit beam used to interrogate
positive wire 410 and line 412 is the transmit beam used to interrogate
wire 414. During actual image formation many lines would be shot at
predetermined angular increments. Lines 400, 404, 408 and 412 represent
only four of these lines.
FIGS. 5A-8B illustrate the intermediate and displayed signals resulting
when the test phantoms shown in FIGS. 4A and 4B are imaged with the prior
art system shown in FIG. 1. More particularly, FIGS. 5A and 5B illustrate
two graphs of "continuous" signal amplitude vs. transmit beam angle for
signals generated by a beamformer circuit for the two test phantoms
illustrated in FIGS. 4A and 4B, respectively. These diagrams represent
theoretical responses which would be expected if an infinate number of
lines were shot. As shown in FIG. 5A, as the transmit angle is adjusted so
that the acoustic line aligns with the wire in the position shown as line
400, the signal amplitude reaches a maximum. There are two maxima, one
corresponding to each of the positive wires shown in FIG. 4A corresponding
to transmit beams 400 and 404. In FIG. 5B, there is a positive maximum and
a negative maximum corresponding to the positive and negative wires shown
in FIG. 4B.
Assuming that the system shown in FIG. 1 is used to process the beamformer
data, FIGS. 6A and 6B show the "continuous" signal which results from the
output of a detector such as detector 106. As previously mentioned, this
detector is an absolute value detector and thus the amplitude of the
signal becomes positive or folded over the axis. In FIG. 6A, the detected
signal is essentially the same as the beamformer output since the original
signal is entirely positive. However, in FIG. 6B, the negative portion of
the signal appears as a second positive maxima due to the squaring action
of the detector.
However, an actual imaging system does not shoot an infinite number of
lines, but uses a finite number of lines to generate the image. The effect
of using a finite number of lines is to convert the signal into a sampled
data signal. FIG. 7A illustrates what such a signal would look like if
only four lines were used to interrogate the test phantom shown in FIG.
4A. The four lines shot correspond to lines 400 and 404 illustrated in
FIG. 4A and two additional lines, shot on either side of lines 400 and
404. FIG. 7B illustrates a four line signal for the phantom shown in FIG.
4B. The two vertical lines in each of FIGS. 7A and 7B correspond to the
acoustic lines which intercept the wires. The sampled signals are
effectively the amplitude of the continuous signals shown in FIGS. 6A and
6B at the sampling angles. Since the continuous signals exhibit only
positive maxima in both FIG. 6A and 6B, the sampled signals in FIGS. 7A
and 7B are exactly the same.
In order to display the sampled signals, a scan conversion is done in which
the sampled version of the signals is linearly interpolated to produce the
final output. This interpolated output is shown in FIGS. 8A and 8B and is
identical for both of the test phantoms. Thus, although the test phantoms
shown in FIGS. 4A and 4B are different, the resulting images are the same
because the phase information has been discarded during the signal
processing procedure.
FIGS. 9A-10B illustrate the intermediate and displayed signals which are
generated when the inventive apparatus shown in FIG. 3 is used to process
the beamformer signals. In particular, the output of beamformer 302 as
shown in FIG. 3 is now directly applied to scan converter 310. In this
case, interpolation takes place before detection. As shown in FIG. 9A, in
the sampled signal version of the continuous signals shown in FIGS. 5A and
5B, both of the positive signal maxima illustrated in FIG. 5A result in
positive samples. However, in FIG. 9B, the positive and negative maxima in
FIG. 6B result in one of the samples being positive while the other sample
is negative due to the negative maximum shown in FIG. 6B.
As shown in FIGS. 10A and 10B, the linearly interpolated output of scan
converter 310 is now different for the two test phantoms. When the signal
shown in FIGS. 10A and 10B is detected resulting in the signals shown in
FIGS. 11A and 11B, the resulting signal includes a minimum 1102 resulting
from the lack of signal between the two wires as illustrated in FIG. 5B.
The output of the detector is displayed in the inventive system.
Thus, as can be seen by comparing FIGS. 11A and 11B with FIGS. 8A and 8B,
reversing the order of scan conversion and detection generates an image
with additional information because the phase information present in the
original object is not lost during processing. It can easily be seen that,
for targets having an arbitrary phase difference of .theta., the depth of
the interpolated null produced by the inventive imaging system is
cos(.theta./2) and that the resulting picture contains all the phase
information present at the beamformer summing node. Another way of stating
this is that, from the nulls displayed in the final image, the phase of
the signals at the beamformer summing node can be mathematically
determined.
A comparison of FIGS. 12 and 13 illustrates the improvement in image
quality which results from using the inventive apparatus and method. In
particular, FIG. 12 shows a magnified image of a section of heart muscle.
The image was generated using a conventional ultrasonic imaging system
sold by Hewlett-Packard Company, 3000 Minuteman Road, Andover, MA 01810,
under the name "PRISM" using a 3.5 MHz ultrasonic frequency and an array
with 128 transducers with a 0.75.degree. line spacing. With this
conventional system, detection occurred before scan conversion as shown in
FIG. 1.
FIG. 13 uses the identical apparatus discussed above with the exception
that detection is performed after scan conversion in accordance with the
system shown in FIG. 3. In FIG. 13, each of the bright muscles is outlined
with a clearly defined dark circle corresponding to the null between
fibers thereby more clearly defining the muscles as compared to the
conventional image as shown in FIG. 12.
Conventionally generated images, such as that shown in FIG. 12, often
exhibit a recurring artifact which shows up as bright radial streaks. It
has been found that these streaks are a result of the scan conversion
interpolation which is performed between actual data points to produce the
final display. More particularly, it has been found that the conventional
linear interpolation scheme used in scan conversion causes a lower
effective gain when a data point necessary for display falls between two
R-.theta. data points from two different acoustic lines. In this case, the
prior art interpolators (such as that illustrated in FIG. 2) construct the
necessary data point by scaling the two available data points by means of
multipliers 202 and 206 where the coefficients, A and B, are selected such
that A+B=1.
A linear interpolation scheme such as this is simple to implement and
results in a smooth interpolation which creates no effective D.C. level
shift. However, in accordance with another aspect of the invention, it has
been found that the theoretical continuous angular response of an object
located between acoustic lines which would result if additional lines were
shot is approximately 1.8 db higher than the response calculated by
linearly interpolating the response data between the two data points. In
particular, in accordance with the invention, an interpolation which uses
values corresponding to a sinc function is preferred instead of linear
interpolation.
A preferred structure is shown in FIG. 14. Input data on line 1400 is
applied directly to multiplier 1402 where it is multiplied by the
predetermined constant A and applied to the summing point 1412. Input 1400
is also applied to a one line buffer 1404 and the output of buffer 1404 is
applied to a multiplier 1406 and, via output 1410, to summing point 1412.
Additional buffers, two of which are shown as buffers 1416 and 1420, may
also be provided. The output of these buffers are provided to multipliers
1418 and 1422, respectively. The outputs of the multipliers are, in turn,
applied to the summing point 1412. However, in the inventive arrangement,
the coefficients A, B, C . . . N do not sum to unity. Instead, the
coefficients are adjusted so that they assume the values of an ideal sinc
function ((sin x)/x).
In particular, in accordance with the invention, the scan conversion is
treated as equivalent to a classic Nyquist sampling-reconstruction
problem. Specifically, it can be shown by Fourier optics that the angular
spatial frequencies in the acoustic signals generated by any transducer
array are absolutely bandlimited. Consequently, as long as the classic
Nyquist criteria are met, it is possible to acoustically sample an object
at discrete angle increments and to reconstruct the resulting image with
an ideal Nyquist filter.
Since such an ideal filter has a sinc function impulse response, the most
accurate reconstruction will occur when a sinc function is used during the
scan conversion for interpolation. Practically, it is not possible to
generate an ideal sinc function response, since this would require an
infinite number of delays and multipliers. However, it has also been found
that it is not necessary for the sinc function interpolator to be
absolutely ideal. Instead, a curve can be used which approximately
corresponds to the main lobe of the sinc function. This curve replaces the
triangle function that is normally used in the prior art.
For example, in order to calculate a data signal at a point centered
between two known data points, it has been found that, instead of the
prior art method of multiplying each of the two data points by 0.5 and
summing the results, the previously-mentioned artifacts can be reduced by
multiplying each data point by 0.58 and summing the results. Since the
coefficients do not sum to one, the inventive interpolation scheme does
introduce a DC level into the signal. However, scan conversion removes the
DC component before detection thereby eliminating any potential problems.
More particularly, the following equation can be used to estimate the image
field at an angle .theta. from a plurality of known data points 1 . . . N:
signal (.theta.)=.SIGMA.a(i,.theta.-.PHI.) signal
(.PHI.+(i-N/2).DELTA..PHI.)
where .DELTA..PHI.=spacing between discrete angles and
.PHI.=.DELTA..PHI.int(.PHI./.DELTA..PHI.) is the largest discrete angle
less than or equal to .PHI.. The function a(i,.theta.-.PHI.) designates a
continuous interpolation function.
As the number of data points N used in the interpolation becomes large, the
interpolation function a(i,.theta.-.PHI.) approaches a sinc function:
##EQU1##
However, if the interpolation is performed using a small number of points,
it is necessary to chose a function a(i,.theta.-.PHI.) empirically. For
example for N=2 (two-point interpolation) the curve shown in FIG. 15 was
found to give satisfactory results and shows the 0.58 value used in the
previous example. When this value was used in the two-point interpolation,
the radial line artifacts present in the prior art image were greatly
reduced.
In accordance with another aspect of the invention, it has been found that
the imaging frame rate can be increased without loss of resolution by
using nonuniform angular sampling. In particular, prior art imaging
systems use uniform angular sampling as shown in FIG. 16 in which the
angular increment .alpha. between acoustic lines is constant over the
entire 180.degree. image sector so that the angle (called a steering
angle) for the nth acoustic line (.PHI..sub.n), .PHI..sub.n =n.alpha.. For
example the angular spacing between transmit lines 1600 and 1602
(schematically illustrated as lines in FIG. 16) is the angle .alpha.. This
angle is the same as the angle, .alpha., between two other lines 1604 and
1606. Thus, the angular spacing is independent of the steering angle,
.PHI.. In accordance with another aspect of the invention, it has been
found that the prior art uniform angular sampling oversamples the object
for large steering angles thereby resulting in unnecessarily low frame
rates. More specifically, for large steering angles, the effective
"aperture" of the phased array can be reduced by the cosine of the
steering angle due to the angles at which the transmit and receive beams
propagate. As the aperture is reduced in size, the effective width of the
transmit and receive means increases. Therefore, a larger angular spacing
between acoustic lines can be used to obtain the same resolution. By
reducing the number of lines shot at large angles, the overall number of
lines can be reduced to obtain an image with a predetermined resolution.
More particularly, it has been found that the acoustic lines can be spaced
on a grid which is uniform in the reciprocal of the cosine of the steering
angle so that the steering angle for the nth acoustic line is:
.PHI.=sin.sup.-1 (N.alpha.). This method results in the acoustic line
spacing shown in FIG. 17 and allows the effective frame rate to be
increased. As shown in FIG. 17, if the spacing between lines 1704 and 1706
is .alpha., then, at large steering angles, .PHI., the angular spacing
between lines is increased by 1/cos.PHI. as the spacing shown between
lines 1700 and 1702 is .alpha./cos.PHI..
In accordance with another aspect of the invention, it is also possible to
reduce the number of acoustic lines, and thereby increase the frame rate,
by, prior to beamforming, synthesizing acoustic information from linear
combinations of the data available from acoustic lines which are shot. In
accordance with Nyquist sampling theory, there exists a maximum angular
spacing between the lines to insure no loss of spatial information. More
particularly, the maximum allowed angular spacing between transmit lines,
.alpha..sub.t, can be derived by using Fourier optics and the spatial
Nyquist sampling theorem and is given by:
##EQU2##
As an example, the latter formula, when used with a conventional
ultrasonic imaging system sold by Hewlett-Packard Company, 3000 Minuteman
Road, Andover, MA 01810, under the name "PRISM" using a 3.5 MHz ultrasonic
frequency and an array with 128 transducers (used for both transmit and
receive) spaced one-half wavelength apart gives .alpha..sub.t =0.90
degrees.
With the above-mentioned system, a receive beam is formed at each transmit
beam angle by delaying and summing signals from a number of receive
elements in a manner previously described. Even though the transmit line
spacing is given by the above equation, the receive lines must separated
by a smaller angular spacing .alpha..sub.r given by:
##EQU3##
For the example given immediately above .alpha..sub.r= 0.45 degrees. This
difference between the required number of transmit and receive lines
allows the use of an inventive method and apparatus in which acoustic
lines are shot at the spacing determined by .alpha..sub.t and
interpolation is performed on the signals from each receive transducer to
synthesize the signals that would have been available if the actual line
spacing had been .alpha..sub.r.
This method works because, prior to beamforming, each individual receive
element acts like its own system with a receive aperture width of
near-zero. Therefore, to adequately sample the signals on each individual
receive element, the transmit angular separation .alpha..sub.t can be used
because .alpha..sub.r becomes equivalent to .alpha..sub.t when the receive
aperture is set to zero width.
The simplest interpolation construction involves the synthesis of two
receive beams for each transmit beam that is actually shot. The synthesis
is performed so that the synthesized beams are "received" on each side of
a central transmit beam as shown in FIG. 18. Two beamformers are used to
reconstruct the signals so that the beamformer outputs appear as if two
transmit beams were shot. FIG. 18 schematically illustrates a portion of a
sector with transmit and receive beams illustrated as lines (the beam
angular spacing is greatly exaggerated in FIG. 18 for clarity). The solid
lines 1800-1808 represent the transmit beams which are actually shot. The
dotted lines represent receive beams which are synthesized from the
received information using circuitry as described below. In accordance
with the invention, the received signals can be used to synthesize two
receive beams as if two transmit beams had been shot even though they
actually were not. For example, synthesized receive beam illustrated as
line 1810 can be generated from the received information from transmit
beam 1802 by means of delays and linear combinations. In a similar manner,
synthesized beam 1812 can also be generated from received information
generated by transmit beam 1802, resulting in a pair of beams identified
by bracket 1814 being generated from a single transmit beam. The
synthesized beams are arranged symmetrically around the transmit beam from
which they are generated. More specifically, if the transmit beams have an
angular spacing of .DELTA..theta., then the received beams are generated
at an angular spacing of .DELTA..theta./4 on either side of the associated
transmit beam. The spacing between the synthesized receive beams and the
transmit beam spaces the synthesized receive beams at equal angular
increments of .DELTA..theta./2. In a similar manner, transmit beam 1804
can be used to generate two synthesized beams identified by bracket 1816.
Synthesized beams identified by bracket 1818 are, in turn, generated from
transmit beam 1806.
In order to synthesize receive beams, the conventional receiving circuitry
must be modified. In a conventional scanning system as shown in FIG. 19, a
plurality of receive transducer elements, designated as elements 1 . . .
N, are used to construct a receive beam. For simplicity, only two elements
1900 and 1902 are shown. Each element is connected directly to a
beamformer 1904 which constructs the receive beam by appropriately
weighting and summing the transducer output signals.
FIG. 20 schematically illustrates circuitry which can be used to synthesize
additional line information from existing transducer receive outputs. Each
of transducer elements (of which elements 2000 and 2002 are shown) is
connected to two beamformers 2025 and 2042 through an interpolation
circuit of which interpolation circuits 2001 and 2003 are shown. As each
of the interpolation circuits are identical, only circuit 2001 will be
discussed in detail. Although only two synthesized beams are generated, in
general, the transducer output information could be used to synthesize
three or more receive lines as will be discussed below. The extension of
the circuitry for three or more lines is straightforward.
More specifically, the output of element 2000 on lead 2004 is provided to a
pair of line-generator circuits; the first circuit consists of multipliers
2008, 2014, and 2020, and summing junction 2022, and the second
line-generator circuit consists of multiplier 2024, 2026, and 2028, and
summing junction 2030. In the first line-generator circuit, output 2004 is
provided directly to multiplier 2008 and to the input of line buffer 2010.
Line buffer 2010 delays the output 2004 for a time period equivalent to
the transmit and receive time of the system so that the output 2012 of
line buffer 2010 comprises the output of transducer 2000 for the previous
acoustic line.
Output 2012 is, in turn, provided to a second line buffer 2016, so that the
output of this latter buffer on lead 2018 consists of the output 2004 of
transducer 2000 delayed by two line time periods. The outputs, 2012 and
2018, of line buffers 2010 and 2016 are respectively provided to
multipliers 2014 and 2020.
Multipliers 2008, 2014, and 2020 are supplied with constants a.sub.1,
a.sub.2 and a.sub.3, respectively, that scale the transducer and line
buffer outputs. Each multiplier provides a scaled output to a summing
junction 2022. The scaling and summing synthesizes a "new" receive value
on the output 2023 of summing junction 2022 from the transducer output
2004 from the receive information available for three consecutive transmit
lines. This synthesized output is provided to one input of a conventional
beamformer 2025.
The output of transducer 2000 on line 2004 and the outputs 2012 and 2018 of
line buffers 2010 and 2016 are also provided to three additional
multipliers: 2024, 2026 and 2028. These latter multipliers are provided
with three different scaling constants, b.sub.1, b.sub.2 and b.sub.3, and
the scaled outputs are applied to summing junction 2030 in order to
generate an additional synthesized output. If the "a" and "b" constants
differ, the second synthesized output will differ from the first
synthesized output. The latter synthesized output on line 2032 of summing
junction 2030 is provided to the first input of a second conventional
beamformer 2042.
Beamformer 2025 generates an output on lead 2027 and beamformer 2042
generates an output on lead 2044. These outputs can be stored and
processed as if twice as many lines were shot than the actual number of
lines.
A similar interpolation circuit is provided for the output of each
transducer element. For example, interpolation circuit 2003 is provided at
the output of transducer element 2002. Each interpolation circuit
generates two synthesized lines. One of these lines is provided to one
input of beamformer 2025 and the other line is provided to one input of
beamformer 2042. For example, the outputs of interpolation circuit 2003
generated by summing junction 2034 and 2038 are provided via lines 2036
and 2040 as the "n" input to beamformer 2025 and 2042.
One problem with the circuit shown in FIG. 20 is that two line-generating
circuits must be connected to each transducer output resulting in a total
of 2N line-generator circuits Consequently, the circuit can be expensive.
FIG. 21 shows an alternative embodiment in which interpolation is performed
after beamforming in order to reduce the number of line-generating
circuits required. In particular, the outputs of the N receive transducer
elements (of which elements 2100 and 2102 are shown) are provided to two
beamformers 2125 and 2142. More particularly, the output of transducer
element 2100 is provided, via lead 2104, to beamformer 2125 and, via lead
2101, to beamformer 2142. In a similar manner, the output of transducer
2102 is provided, via lead 2103, to beamformer 2125 and, via lead 2105, to
beamformer 2142.
The output of each of the beamformers 2125 and 2142 is, in turn, provided
to an interpolation circuit. For example, the output of beamformer 2125 on
lead 2127 is provided to interpolation circuit 2150. In a similar manner,
the output 2144 of beamformer 2142 is provided to interpolation circuit
2152. As interpolation circuits 2150 and 2152 are essentially equivalent,
only interpolation circuit 2150 will be described in detail.
Interpolation circuit 2150 consists of two line buffers 2154 and 2156,
three multipliers 2162-2164 and a summing junction 2166. Multiplier 2160
multiplies the output of beamformer 2125 by a predetermined constant
a.sub.1 and provides the scaled output to summing junction 2166. The
output of beamformer 2125 is also applied to line buffer 2154 which, as
previously described, delays the output for a time period equal to the
time necessary to shoot one acoustic line. The output of line buffer 2154
on lead 2158 is provided to multiplier 2162 wherein it is multiplied by a
second constant a.sub.2 and applied to summing junction 2166. The output
of line buffer 2154 on lead 2158 is also provided to line buffer 2156
where it is delayed by another time period equal to an acoustic line time
duration. The output of line buffer 2156 is, in turn, applied to
multiplier 2164 where it is multiplied by a constant a.sub.3. The scaled
output provided to summing junction 2166.
By suitably adjusting the constants a.sub.1 -a.sub.3, a sum can be formed
at the output 2168 of summing junction 2166 which is the interpolated
output of beamformer 2125 constructed from three successive acoustic line
scans.
Interpolator 2152 operates in a similar manner to generate a second
interpolated output on lead 2170. The constants and the multipliers in
interpolator 2152 are adjusted to the same values of the multipliers in
interpolator 2150. This scheme functions in a similar manner to that shown
in FIG. 20 with the exception that only two interpolation circuits are
necessary instead of the 2N interpolation circuits necessary in FIG. 20.
When two receive beams are synthesized for each transmit beam, there will
be a signal-to-noise ratio loss because the synthesized transmit beams do
not return along the path taken by the transmit beam as shown in FIG. 18.
There also may be a "checkerboard" artifact produced since all synthesized
receive lines don't have identical beam profiles. In order to eliminate
the signal-to-noise penalty and possible artifacts, three beamformers can
be used to generate three outputs from the received data from each actual
transmit beam. The beamformer outputs are preferably generated at the
sequence of angles given in the following table for each transmit angle:
TABLE 1
______________________________________
Trans-
mit Beamformer #1
Beamformer #2
Beamformer #3
Angle receive angle
receive angle
receive angle
______________________________________
.cndot.
.cndot. .cndot. .cndot.
.cndot.
.cndot. .cndot. .cndot.
.cndot.
.cndot. .cndot. .cndot.
0 0 -.DELTA..theta./2
.DELTA..theta./2
.DELTA..theta.
.DELTA..theta.
.DELTA..theta./2
3.DELTA..theta./2
2.DELTA..theta.
2.DELTA..theta.
3.DELTA..theta./2
5.DELTA..theta./2
3.DELTA..theta.
3.DELTA..theta.
5.DELTA..theta./2
7.DELTA..theta./2
.cndot.
.cndot. .cndot. .cndot.
.cndot.
.cndot. .cndot. .cndot.
.cndot.
.cndot. .cndot. .cndot.
______________________________________
In order to synthesize round-trip receive line information, the outputs of
each beamformer are stored in a memory and the stored outputs are then
combined to generate the synthesized receive beams. A preferred
combination is given in Table II:
TABLE II
______________________________________
Synthesized
round-trip Linear combination used to
angle synthesize round-trip beam
______________________________________
.cndot. .cndot.
.cndot. .cndot.
.cndot. .cndot.
0 R1(0)
.DELTA..theta./2
0.68*[R2(.DELTA..theta.) + R3(0)]
.DELTA..theta. R1(.DELTA..theta.)
3.DELTA..theta./2
0.68*[R2(2.DELTA..theta.) + R3(.DELTA..theta.)]
2.DELTA..theta. R1(2.DELTA..theta.)
.cndot. .cndot.
.cndot. .cndot.
.cndot. .cndot.
______________________________________
where Rn(x) is the stored output signal generated by beamformer n from a
transmit beam at steering angle x. An examination of Table II indicates
that the synthesized round-trip receive beam data is generated by
averaging data from transmit beams shot at two different steering angles.
Effectively, the combination of data from two transmit beams makes the
system appear as if a third transmit beam was actually shot between actual
transmit beams.
The synthesized line information is illustrated in relation to the original
transmit beams in FIG. 22. As with FIG. 18, the actual transmit beams are
shown in FIG. 22 as solid lines 2200-2208. The synthesized receive beams
are shown in dotted lines. In accordance with Table II, the data from two
transmit beams is used to synthesize one of the receive beams. For
example, a receive beam 2210 is generated from data from transmit beam
2200 and receive beam 2214 is generated from data from transmit beam 2202.
Receive beam 2212 is generated by combining data from transmit beams 2200
and 2202. In a similar manner, receive beams 2218, 2222 and 2226 are
generated from transmit beams 2204, 2206 and 2208, respectively. Receive
beams 2216, 2220 and 2224 are generated from transmit beams pairs 2202,
2204; 2204, 2206 and 2206, 2208, respectively. Brackets 2228, 2230 and
2232 identify receive beam information for groups of three beams which are
generated in parallel.
In this latter synthesis, there is no loss in signal-to-noise ratio since
the synthesized receive beams are in perfect alignment with either actual
transmit beams or "synthesized" transmit beams. In fact, there is a slight
signal-to-noise ratio gain due to a resolution - signal-to-noise ratio
tradeoff. However, as in the two parallel beam scheme, there may be a
"checkerboard" artifact since all round-trip beams don't have the same
beam profile. In addition, this scheme may be sensitive to object motion
since it averages data generated by transmit lines shot at different
times.
It is also possible to use four parallel beamformers to generate four
parallel outputs at the transmit and receive angles shown in Table III:
TABLE III
______________________________________
Beamfmr
Transmit
Beamfmr#1 Beamfmr#2 Beamfmr#3
#4
Angle rcv angle rcv angle rcv angle
rcv angle
______________________________________
.cndot.
.cndot. .cndot. .cndot. .cndot.
.cndot.
.cndot. .cndot. .cndot. .cndot.
.cndot.
.cndot. .cndot. .cndot. .cndot.
0 -3.DELTA..theta./4
-.DELTA..theta./4
.DELTA..theta./4
3.DELTA..theta./4
.DELTA..theta.
.DELTA..theta./4
3.DELTA..theta./4
5.DELTA..theta./4
7.DELTA..theta./4
2.DELTA..theta.
5.DELTA..theta./4
7.DELTA..theta./4
9.DELTA..theta./4
11.DELTA..theta./4
3.DELTA..theta.
9.DELTA..theta./4
11.DELTA..theta./4
13.DELTA..theta./4
15.DELTA..theta./4
4.DELTA..theta.
13.DELTA..theta./4
15.DELTA..theta./4
17.DELTA..theta./4
19.DELTA..theta./4
.cndot.
.cndot. .cndot. .cndot. .cndot.
.cndot.
.cndot. .cndot. .cndot. .cndot.
.cndot.
.cndot. .cndot. .cndot. .cndot.
______________________________________
As in the previous synthesis methods, the outputs of each beamformer are
stored in memory and the stored outputs are subsequently pieced together
in a linear combination in the manner described in Table IV to synthesize
round-trip receive lines:
TABLE IV
______________________________________
Synthesized
round-trip Linear combination used to
angle synthesize round-trip beam
______________________________________
.cndot. .cndot.
.cndot. .cndot.
.cndot. .cndot.
.DELTA..theta./4
.93*R3(0) + .28*R1(.DELTA..theta.)
3.DELTA..theta./4
.28*R4(0) + .93*R2(.DELTA..theta.)
5.DELTA..theta./4
.93*R3(.DELTA..theta.) + .28*R1(2.DELTA..theta.)
7.DELTA..theta./4
.28*R4(.DELTA..theta.) + .93*R2(2.DELTA..theta.)
9.DELTA..theta./4
.93*R3(2.DELTA..theta.) + .28*R1(3.DELTA..theta.)
.cndot. .cndot.
.cndot. .cndot.
.cndot. .cndot.
______________________________________
where Rn(x) is the stored output signal generated by beamformer n while
from data received from a transmit beam shot at steering angle x. This
combination results in the synthesized beams shown schematically in FIG.
23.
As before, the actual transmit beams are schematically illustrated as solid
lines and the synthesized receive beams are shown as dotted lines. In the
latter method, all receive beams are synthesized from two transmit beams.
For example, receive beams 2304 and 2306 are synthesized from data
received from transmit beams 2300 and 2302. Brackets 2308 and 2310
identify indicate groups of parallel receive beams synthesized from
transmit data. As with the previous three beam method, there is a slight
signal-to-noise ratio gain and some sensitivity to object motion. However,
the four beam method has an advantage that all synthesized beams have
virtually identical beam profiles for all round-trip angles and therefore
there will not be a "checkerboard" artifact.
Although only a few embodiments of the inventive method and apparatus have
been described, several modifications and changes will be immediately
apparent to those skilled in the art. These modifications and other
obvious changes are intended to be covered by the following claims.
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